Search

Premium Membership ♕

Experience matters. Premium membership gives you an opportunity to learn from experienced electrical engineers. Enjoy studying from low to high voltage!

High-pass filter at generator terminals

This example is aimed at illustrating the reduction of THDV using a high-pass filter in an installation. High-pass filter is used to control a resonant condition. The example describes another real-world application in which harmonic-related problems arose and even damaged a generator unit. This case presented an excellent opportunity for conducting investigations on a complicated phenomenon.

The essentials of harmonic filtering techniques
The essentials of harmonic filtering techniques (on photo: ABB drive panel with bypass and harmonic filter; credit: u/stoned_jack_baller via Reddit)

The AC source, a 3-MVA synchronous turbo generator, was the power supply for two 1.5 MVA step-down transformers, which in turn fed a number of VFDs powering downhole electrosubmersible pumps. Apart from a small service transformer that sourced the platform services, the VFDs were the only loads as illustrated in Figure above.

A number of harmonic measurements indicated that voltage and current harmonic levels were excessively high. The suspecting element was a 0.27-µF surge protection capacitor bank, which apparently combined with connecting cables between generator and transformers to excite a resonant condition at the generator bus.

Through additional measurements and simulation work, this was further confirmed to be the case.

A high-pass filter at the generator bus on an offshore oil field installation
Figure 1 – A high-pass filter at the generator bus on an offshore oil field installation

Here, we will show the resonant condition and how it was controlled using a high-pass filter to reduce harmonic distortion levels at the generator terminals within IEEE-519 limits.

Figure 2 shows the abrupt increase in impedance (dotted line) revealing a resonant condition around harmonics 39 through 43.

Such a situation imposed severe stresses on the generator that comprised intense shaft vibration and increased operation temperature. Also shown is the Z–f characteristic of the band-pass filter that was applied at the 600-V generator bus. Filter elements were selected to obtain a corner frequency centered at around the 11th harmonic.

Impedance vs. frequency characteristics at the generator bus
Figure 2 – Impedance vs. frequency characteristics at the generator bus

Notice the system response with the high-pass filter showing a substantial reduction of the impedance at the resonant point.

Figure 3 shows how the impedance frequency characteristic looks at the primary of one of the downstream transformers feeding a VFD at one of the oil wells.

At these locations, 5th-, 7th-, and 11th-harmonic filters were installed. This is noticeable on the Z–f characteristic, which also reflects the band-pass filter effect on the distribution system as seen from that location. Thus, the installation of a low-cost high-pass filter (involving a 67-kVA capacitor bank) at the generator bus allowed a very annoying condition to be brought under control.

Interestingly, the resonant phenomenon, as observed in Figure 6.14, involved a frequency range in which characteristic harmonics show very small (but apparently strong enough) values that, after undergoing amplification, made THD levels soar.

Impedance vs. frequency characteristics at a downstream VFD
Figure 3 – Impedance vs. frequency characteristics at a downstream VFD

It is important to mention that applications of high-pass filters at the generator bus are usually combined with the application of single-tuned filters at the VFD locations to get THD levels within recommended limits at the generator terminals as well as at the individual VFD sites.

Leaving the high-pass filter at the generator bus as the only harmonic mitigating method may cause excessive heating on the high-pass filter elements.

In this example, the real intention of the high-pass filter was to eliminate the large impedance resulting from the resonant condition more than to provide a low-impedance path to all harmonic currents generated at the various VFD sites.

Title:The essentials of harmonic filtering techniques – Francisco c. De La rosa
Format:PDF
Size:1.6 MB
Pages:38
Download:Right here | Video Courses | Membership | Download Updates
The essentials of harmonic filtering techniques
The essentials of harmonic filtering techniques

Premium Membership

Get access to premium HV/MV/LV technical articles, advanced electrical engineering guides, papers, and much more! It will help you to shape up your technical skills in your everyday life as an electrical engineer.
50% Discount 💥 - Save 50% on all 90+ video courses with Enterprise Membership plan.

More Information

One Comment


  1. Reuben paul
    Oct 31, 2020

    Its so awesome

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

two  +    =  seven

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

EEP Hand-Crafted Video Courses

Check more than a hundred hand-crafted video courses and learn from experienced engineers. Lifetime access included.
Experience matters. Premium membership gives you an opportunity to study specialized technical articles, online video courses, electrical engineering guides, and papers written by experienced electrical engineers.